WASHINGTON — US regulators on Tuesday approved Kalydeco, a new, gene-targeted drug treatment for people who have a rare kind of the incurable lung disease cystic fibrosis.

Made by the Massachusetts-based Vertex Pharmaceuticals, Kalydeco (ivacaftor) was approved by the US Food and Drug Administration on a fast-track for drugs that may offer big advances over standard therapy.

The drug works to restore the balance of salt and water on the surface of the patient's airways, a process that is crucial for CF patients because their bodies produce a thick, sticky mucus which makes it hard to breathe.

However it is only for people who have a specific G551D gene mutation, or about 1,200 (four percent) of the 30,000 people in the United States who have cystic fibrosis. Some 70,000 people worldwide suffer from the disease.

More than 90 percent of CF patients have a different genetic basis, or two copies of the F508 mutation in the CFTR gene. The drug does not work for those people, the FDA said.

The Cystic Fibrosis Foundation nevertheless hailed the approval as an "important milestone in our journey to find a cure for cystic fibrosis," said the group's president, Robert Beall.

"The drug has opened exciting new doors to research and development that may eventually lead to additional therapies that will benefit more people living with CF."

A phase III trial published in November 2011 in the New England Journal of Medicine was hailed as a promising breakthrough after results showed the oral twice-daily medication could improve lung function, growth and sweat chloride levels over a span of 11 months.

The clinical trial involved 213 patients, some as young as six years old. Results showed "significant and sustained improvement in lung function," the FDA said.

It remains unclear if longer-term exposure to the drug could halt the deterioration of lung function.